Well, for quantum foam I really mean to use the technical term "awesomesauce", or perhaps "world lubricant", but that sounds odd. Don't call it that.
I think what I mean is that there's sometimes a space in a game where the "authority structure" (whether that leans toward GM authority, or games with a more collaborative approach) does, as
@chaochou says, break down. Suddenly no one knows if there are any pine cones on the forest floor, or if this particular wooden frame's made of oak or ash, or if these are the same goat-herders you saw from a distance three sessions ago.
By immediate authority I mean details that are perhaps too zoomed in, too fine grained, or just too ephemeral/mundane for the GM to have put in their prep. So the GM can't look at their notes to find out. The players have asked questions that do need an answer, and everyone is caught in a moment where it's Schroedinger's X. It's like a rugby scrum, in a way, two teams leaning into each other, balanced, with the ball bouncing around somewhere inside. An observer knows the ball is there, but can't say where.
Instead of this being a failure state, point of contention, or opportunity for the GM to re-assert authority ("Give me ten minutes while I research the types of wood used in Norse door frames and the density of pine cones in mixed forests"), it can be a fantastic opportunity to explore the emergent properties of a setting. Burning Wheel does this with the Die of Fate. Questions from the GM and players that can be formulated along the lines of could it be the case that...? is it possible and logical that X might happen...? are resolved by rolling a d6. On a 1, it happens. Maybe the odds get adjusted, if the group thinks that 2-in-6 or 50/50 is a better probability.
Here the immediate authority of the GM has broken down, the players can only ask questions of the environment, so the answer to the question is ceded to the die roll. And the game (not just Burning Wheel, any game can add this mechanic in with a coin toss or whatever), the fiction, the world-in-motion is taken out of the hands of the group. Many of the most memorable tangents and zig-zags and out-of-left-fields in the games I run have happened because of this "quantum foam", as I call it.
You feel me fam? Or am I talking rubbish?